Digital image processing falls broadly into two fields of application, namely improving pictorial information for interpretation by the eye, and processing a digital signal representative of an image by applying signal processing techniques to it. The present application is concerned with image signal-processing techniques typically implemented in software. Image processing techniques are generally based on Fourier transforms which transform signals between the time or spatial domain to the frequency domain. In mathematically processing image signals it is more convenient to operate in the frequency domain. In filtering a digital image the filter may be applied to the image by convolution in the time domain. This convolution is mathematically demanding but can be performed more simply in the frequency domain by using the relevant Fourier transforms.